The present invention relates to a high-performance active wide-range PLL device used for an optical disk reproducing apparatus or the like such as a CD (Compact Disk) or CD-ROM driving system.
At present, in the field of acoustic equipment, a digital recording/reproducing system (disk reproducing apparatus) is known, which converts an audio signal into a digital signal by a PCM (Pulse Code Modulation) technique, and recording/reproducing this signal on/from a recording medium such as a disk or a magnetic tape so as to perform a high-density, high-fidelity recording/reproducing operation. CDs have been used most widely. A CD is a disk having a diameter of 12 cm on which bit strings corresponding to digital data are formed, and from which the bit strings are optically read. Such a disk reproducing apparatus is designed to move an optical pickup element incorporating a semiconductor laser or a photoelectric conversion element from the inner periphery side to the outer periphery side in the manner of linear tracking, and rotate a CD at a constant linear velocity (CLV) so as to read data recorded on the CD.
A CD-ROM driving system is a typical disk reproducing apparatus. The CD-ROM driving system is an apparatus for reproducing an audio signal and ROM data such as image information and a character code which are recorded together on a disk. The audio signal is reproduced at a normal reproduction speed (single speed) to output a sound. In contrast to this, the ROM data is reproduced at the double speed or higher to read the data as fast as possible. When the disk having such an audio signal and ROM data recorded together thereon is to be played, the reproduction speeds must be frequently switched (e.g., from the single speed to the double speed, and vice versa).
A CD system was originally an audio reproducing system, and hence was rotated at the single speed. However, ever since the CD system was used as a CD-ROM driving system, the drawback of this system has been the low transfer rate as compared with general computer storage media. Furthermore, in consideration of the reproduction processing of current large-capacity moving image data, it is important to increase the transfer rate. Recently, quadruple-speed reproducing apparatuses have been taken for granted, and the reproduction speed has increased from the quadruple speed to the sextuple speed and the octuple speed; there is no end to speed competition.
In general, however, the disk specifications of a CD are determined on the basis of the single speed, and constant linear velocity recording (CLV recording) is performed. For this reason, the number of revolutions of the disk differs at the inner and outer peripheries. In the single speed mode, the inner periphery of the disk rotates at 8 Hz; and the outer periphery, at 3 Hz. In the double speed mode, the inner periphery rotates at 16 Hz; and the outer periphery, at 6 Hz. In the octuple speed mode, the inner periphery rotates at 64 Hz; and the outer periphery, at 24 Hz. That is, the disk must be rotated at very high speeds. In addition, an expensive, high-performance motor with a larger torque is required for a seek operation because this number of revolutions must be corrected within a short period of time.
Recently, however, as the rotational speed of a disk has approached its limit, a driving apparatus designed to perform CAV (Constant Angular Velocity) control to rotate a disk at a constant angular velocity has been developed. In this system, the number of revolutions of the disk motor need not be changed. In the CAV system, however, the transmission rate of data varies. For example, when the disk is rotated such that the data transfer rate at the inner periphery becomes the quadruple speed, the transfer rate at the outer periphery becomes 2.5 times that at the outer periphery at the same number of revolutions. In addition, in the CAV system, a mechanical part such as an FG (Frequency Generator) serving as a circuit for generating pulses in accordance with the number of revolutions of the motor is required to rotate the motor at a constant speed. In addition, since the reproduction speed (data transfer rate) changes from the inner periphery to the outer periphery, the frequency range in which the PLL can be locked must be expanded. Furthermore, CAV control must be switched to CLV control to reproduce normal musical sounds.
As described above, in the case of CAV control, the data transfer rate at the outer periphery becomes twice or more that at the inner periphery. In addition, since data are sequentially recorded on most dicks from the inner periphery to the outer periphery, data tracks concentrate on the inner periphery side of the disk. For this reason, in the case of CAV control, the system often operates at low transfer rates, resulting in disadvantages.
If, however, data can be read by rotating a disk at a constant angular velocity (CAV), the motor required to rotate the disk need not have a very large capacity. In addition, since rotation variations need not be corrected even in a seek time, the system can operate at higher speeds. In order to realize a data read operation by rotating the disk under CAV control, the variable width of the VCO circuit in the disk driving system may be increased. When a disk on which data is recorded at a constant linear velocity is rotated at a constant angular velocity, the data rate of signals extracted from the inner periphery differs from that of signals extracted from the outer periphery by 2.5 times. That is, the transfer rate at the outer periphery is 2.5 times higher than that at the inner periphery. In order to read this data, therefore, the capture range of the VCO circuit must be expanded. In other words, the oscillation range of the VCO circuit must be expanded.
To increase the variable width of the VCO circuit is to increase the conversion gain of the VCO circuit and the gain of the PLL. In general, as the gain of the PLL increases, the response speed of the PLL in a closed state increases to follow a slight change in input signal with time. As a result, the jitter in a sync clock CK deteriorates, and the error rate of data increases. The variable width of the VCO circuit must be increased to shorten the seek time. However, this increase is not preferable in consideration of the reliability (the error rate of data) of the CD-ROM driving system. In the normal reproduction state, a variable width of several % is sufficient for the oscillation frequency at each lock position of the PLL. Assume that the disk is rotating at a perfectly constant angular velocity. In this case, in order to absorb variations in data rate in a seek operation, very wide variable widths of -60% and +260% are respectively required when the seek operation is performed from the outer periphery to the inner periphery, and from the inner periphery to the outer periphery. For this reason, the condition for this normal reproduction mode contradicts the condition for the optimal seek time in the high-speed reproduction mode; it is very difficult to satisfy both the conditions.